(a) Field of the Invention
The present invention relates to a silicon derivative, a liquid crystal composition comprising the same, and a compensation film for a liquid crystal display device using the same liquid crystal composition. More particularly, the present invention relates to a liquid crystal material of a high-quality view angle compensation film, which improves a contrast ratio measured at a tilt angle when compared to a contrast ratio measured from the front surface and minimizes color variations in a black state depending on view angles. The present invention also relates to a liquid crystal composition comprising the same liquid crystal material and a compensation film obtained from the same liquid crystal composition.
(b) Description of the Related Art
Recently, as watches, notebook PCs, cellular phones, televisions and monitors have extended the market, display devices having low weight and requiring low power consumption have been increasingly in demand. Since liquid crystal display devices (LCDs) are light and thin and require low power consumption, they have been widely applied to such products.
However, a liquid crystal display device has a disadvantage of view angle dependency. In other words, an LCD shows variations in color or light/darkness depending on view directions or angles. Additionally, as the screen of an LCD increases in size, view angle decreases more and more. As compared to a conventional CRT (cathode ray tube) device having a view angle of about 180°, a TFT-LCD with no view angle compensation shows a view angle of merely about ±50°.
To solve the above problem, various methods have been used, such methods including a multi-domain method in which pixels are divided in liquid crystal cells to control the liquid crystal alignment, a method of controlling a voltage and a method of utilizing an optical compensation film.
The above-mentioned view angle dependency of a liquid crystal display device is caused by the incident light having a tilt angle to the LCD panel, which shows a birefringence effect different from that of the vertical incident light. To compensate for this, an optical compensation film has been widely used. Herein, retardation films having an opposite birefringence index to the panel are attached onto both surfaces of the panel. Also, as display panels have increased in size, there has been a need for a high-quality liquid crystal compensation film.
A retardation film is obtained by coating an aligned transparent support with liquid crystal, and aligning the liquid crystal along a predetermined direction to the direction of an aligning layer, followed by curing. After aligning, the liquid crystal has a direction opposite to the direction of liquid crystal cells upon application of a voltage, so that light leakage in a black state can be minimized. When combining such retardation films with a liquid crystal panel and light is allowed to penetrate through the panel, it is possible to compensate for a retardation of light caused by a difference of light paths, because the incident light has a similar path in all directions. Additionally, it is also possible to perform compensation of a difference in birefringence indexes in all directions by optimizing a birefringence latitude of each film, an angle formed between films, a rubbing direction and an angle to a polarizer.
A liquid crystal compound used to manufacture such films should be stable against moisture, light, heat, air, electric fields, or the like. Also, the liquid crystal compound should be chemically stable under an environment of use. Further, in order to apply a liquid crystal compound to a display device, the compound should have adequately balanced physical properties, including a broad range of liquid crystal phase temperatures and refraction anisotropy (Δn). Under these circumstances, there is a need for a novel liquid crystal compound satisfying various physical properties required for various types of display devices.